Engines combust a mixture of air and fuel (air/fuel) to drive a piston in a cylinder. The downward force of the piston generates torque. A throttle controls air flow delivered to the cylinders. By determining the amount of air ingested by the cylinders, the fuel mass can be calculated and a proper air/fuel mixture can be delivered to the cylinders to obtain the desired air-fuel ratio and torque.
Air flow delivered to the cylinders can be measured using a mass air flow (MAF) sensor. The MAF sensor measures the air flow across the throttle. During steady-state air flow conditions, the air flow measured across the throttle provides an accurate estimation of the fresh air flow delivered to the cylinders. Because the MAF sensor measures air flow across the throttle and not the air into the cylinders, it is most accurate during steady-state conditions, and is less accurate during transient conditions (e.g., when additional air must flow across the throttle to increase the manifold absolute pressure (MAP), or when the mass of airflow must be reduced to reduce the MAP).
Air flow can be estimated using a speed density calculation, which is typically based on MAP, engine RPM, as well as intake air temperature and pressure. The speed density calculation is only an approximation that is valid as tong none of the parameters that are not explicitly accounted for in the calculation varies. However, because the not accounted for parameters do vary over a period of time while driving the vehicle, the speed density calculations are only accurate for a short period of time and need to be adjusted over time. In order to maintain the accuracy of the speed density calculations during transient conditions, the MAF sensor is used during stead state conditions to correct speed density calculation.
In engines without variable cam phasing (VCP) or variable cam timing (VCT), if the mass of fresh air entering the cylinder changes (i.e., is transient) there is a corresponding increase or decrease in MAP. This indicates that the mass of air is either being accumulated or depleted in the intake manifold. During such transient conditions, the speed density calculation is used to determine the mass air flow entering the cylinders. The determination of whether the mass air flow is steady-state or transient can be made by means such as that described in commonly assigned U.S. Pat. No. 5,423,208, the disclosure of which is incorporated herein by reference. The control module uses the appropriate method of estimating the mass air flow into the cylinder based on the air flow state.
However in engines with VCP or VCT, changes in cam position can occur without changing the MAP while causing the MAF sensor reading to change by a large amount. This occurs because the VCP or VCT system allows varying amounts of residual exhaust gas back into the intake manifold, which replaces the fresh air mass in the manifold. As a result, more or less air flows through the throttle and the air flow is transient. Traditional air flow transient/steady-state detection methods, like that disclosed in U.S. Pat. No. 5,423,208 will see no change in MAP and incorrectly determine that the air flow is steady-state.